1. Field of the Invention
The invention relates generally to the field of digital communications. The invention relates more specifically to intercommunication between asynchronous systems.
2. Cross Reference to Related Copending Applications
The present application is related to the following copending U.S. patent applications:
(a) Ser. No. 07/670,289 entitled "SCANNABLE SYSTEM WITH ADDRESSABLE SCAN RESET GROUPS", by Robert Edwards et al, which was filed Mar. 15, 1991 and which issued Dec. 14, 1993 as U.S. Pat. No. 5,271,019; said application being assigned to the assignee of the present application; and
(b) Ser. No. 07/814,389 entitled "METHOD AND APPARATUS FOR MAINTAINING DETERMINISTIC BEHAVIOR IN A FIRST SYNCHRONOUS SYSTEM WHICH RESPONDS TO INPUTS FROM NONSYNCHRONOUS SECOND SYSTEM", by James P Millar et al, which was filed Dec. 26, 1991 and which issued Mar. 22, 1994 as U.S. Pat. No. 5,297,276; said application being assigned to the assignee of the present application.
The disclosures of said copending applications are incorporated herein by reference.
3. Cross Reference to Related Patents
The following U.S. Patents are assigned to the assignee of the present application and are further incorporated herein by reference: (A) U.S. Pat. No. 4,244,019, DATA PROCESSING SYSTEM INCLUDING A PROGRAM-EXECUTING SECONDARY SYSTEM CONTROLLING A PROGRAM-EXECUTING PRIMARY SYSTEM, issued to Anderson et al, Jan. 6, 1981; (B) U.S. Pat. No. 4,752,907, INTEGRATED CIRCUIT SCANNING APPARATUS HAVING SCANNING DATA LINES FOR CONNECTING SELECTED DATA LOCATIONS TO AN I/O TERMINAL, issued to Si, et al. Jun. 21, 1988; (C) U.S. Pat. No. 4,819,166, MULTI-MODE SCAN APPARATUS, issued to Si et al Apr. 4, 1989; (D) U.S. Pat. No. 4,661,953, ERROR TRACKING APPARATUS IN A DATA PROCESSING SYSTEM, issued to Venkatesh et al, Apr. 28, 1987; (E) U.S. Pat. No. 4,835,728 DETERMINISTIC CLOCK CONTROL APPARATUS FOR A DATA PROCESSING SYSTEM, issued to Si, et al., May 30, 1989; and (F) U.S. Pat. No. 4,855,616, APPARATUS FOR SYNCHRONOUSLY SWITCHING FREQUENCY SOURCE, issued to Wang et al., Aug. 8, 1989.
4. Description of the Related Art
Special circuits and dedicated communications are conventionally used to process requests and responses between independently clocked systems. An example of this is seen in the so-called "console immediate control" signals that are exchanged between a mainframe computer and a system-supervising unit. Supervisory communications need to be exchanged at relatively rapid rates and supervisory commands often need to be executed as soon as possible.
A problem develops, however, when each of the mainframe computer and a system-supervising unit is synchronized to its own independent clock. The responses from the request/command receiving machine tend to be asynchronous relative to the clock and commands of the request/command sending machine and vice versa. The exchange of supervisory requests (or commands) and/or responses between the independently clocked systems need to be synchronized to the clock of the receiving unit as the commands or responses cross the boundary from the sending unit into the receiving unit. Given this, a dedicated and high-priority communications channel is conventionally provided to handle system supervisory executions and data exchanges.
The conventional approach suffers from several drawbacks. (1) Supervisory data exchanges often need to take place between a central system-supervising unit and many different and relatively far-apart circuits of the mainframe unit. As such, a space-consuming network composed of dedicated cables or interconnect lines has to be distributed throughout the mainframe unit. (2) The expansive and dedicated nature of the supervisory signal exchange network leads to a consumption of numerous interconnect pins on box connectors, printed-circuit board connectors and/or integrated circuit package connectors. The number of interconnect pins available on each type of unit are limited by physical considerations, and as such, each time an interconnect pin is dedicated for use within the supervisory signal exchange network, there is one less pin available for carrying nonsupervisory signals. (3) Dedicated memory devices are often used to hold system-supervising commands and execution results. These dedicated memory devices take away circuit space that could be otherwise used for user-function memory or other user-support functions.